(1) Field of the Invention
The present invention relates to a method of and apparatus for controlling a combustion state in a natural draft type heating furnace for heating a fluid to be heated, which flows through the furnace, in a heat exchanger by heat exchange from a combustion gas, wherein the feed amount of air for combustion is controlled to maintain a good and safe combustion state.
(2) Description of the Prior Art
In a natural draft type heating furnace which is broadly used in a petrochemical plant or the like, the feed amount of air for combustion, which is naturally introduced into the furnace from an air intake opening of a burner, is minutely adjusted by controlling the opening degree of a damper attached to an exhaust gas discharge duct (opening) of the heating furnace.
In this natural draft furnace, when the amount of air fed into the furnace is unnecessarily large, the interior of the furnace is cooled by air not used for combustion and as fuel consumption is increased, an economical disadvantage is brought about. Accordingly, it is preferred that air for combustion be fed in an amount required for heating a fluid to be heated. For this purpose, there has been adopted a method in which the pressure of the combustion gas in the furnace (the draft pressure) is measured and the opening degree of the damper is controlled to maintain the draft pressure at a slightly negative level approximating that of atmospheric pressure so that air can be fed in a minimum required amount. In the draft furnace, however, the draft pressure is locally increased or decreased. Therefore, the draft pressure at the measuring part does not always correspond to the amount of air i.e., the amount of oxygen in the furnace. Accordingly, even if the measured value of the draft pressure indicates an appropriate feed amount of air, it often happens that incomplete combustion takes place because of a shortage of the amount of oxygen throughout the furnace. In this case, if the feed amount of air is instantaneously increased, explosive combustion is caused and there is a risk of damage to or destruction of the furnace. Accordingly, if the amount of oxygen is thus insufficient, the opening degree of the damper should be temporarily increased.
For control of the opening degree of the damper in the conventional draft furnace, there have been adopted a method in which the opening degree of the damper is manually controlled appropriately according to the indicated pressure while watching values on a draft pressure gauge (in many cases, a pressure indicating meter is disposed in a control room and remote control is performed in this room) and a method in which the opening degree of the damper is automatically controlled to a predetermined level in response to the draft pressure detected by a draft pressure sensor.
The former manual control method is defective in that control precision is low and economical utilization of the fuel cannot be sufficiently attained. Further, an operator must always watch the draft pressure gauge and the control operation becomes tiring and difficult. In the latter control method, since the opening degree is controlled only on the draft pressure, control precision is similarly insufficient and since it is very difficult to cope with a temporary shortage of the amount of oxygen in the furnace, the method involves a problem as regards the safety of the operation. Therefore, this method is hardly adopted actually in the art.